linux-kernel - Re: Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

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Message-ID: <20240322155529.GE5634@willie-the-truck>
Date: Fri, 22 Mar 2024 15:55:29 +0000
From: Will Deacon <will@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Robin Murphy <robin.murphy@....com>, Tyler Hicks <code@...icks.com>,
	Jerry Snitselaar <jsnitsel@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
	Easwar Hariharan <eahariha@...ux.microsoft.com>
Subject: Re: Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

Hey Jason,

On Tue, Mar 19, 2024 at 02:50:07PM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 19, 2024 at 03:47:56PM +0000, Will Deacon wrote:
> 
> > Right, it's hard to win if DMA-active devices weren't quiesced properly
> > by the outgoing kernel. Either the SMMU was left in abort (leading to the
> > problems you list above) or the SMMU is left in bypass (leading to possible
> > data corruption). Which is better?
> 
> For whatever reason (and I really don't like this design) alot of work
> was done on x86 so that device continues to work as-was right up until
> the crash kernel does the first DMA operation. Including having the
> crash kernel non disruptively inherit and retain the IOMMU
> configuration. (eg see translation_pre_enabled() stuff in intel
> driver)

Right, I'm also not thrilled about trying to implement that :)
What we have at the moment seems to be good enough to avoid folks
complaining about it.

For the case Tyler is reporting, though, I _think_ it's just a standard
kexec() rather than a crashkernel.

Will